Week Number Calculator
Find the ISO week number for any date, or determine which date a given week number starts on.
Our free Week Number Calculator lets you find the ISO week number for any date, or determine which date a given week number starts on. It is built for project managers, HR teams, travelers, parents, and event planners who need fast, reliable results without installing software or creating an account.
Week Number Calculator runs entirely in your browser on CalculatorsPlus — enter your values, get instant results, and copy or share your output in one click. Your data never leaves your device; we do not store inputs on any server.
Calendar math errors are common when counting manually — especially across month boundaries, leap years, and weekends. The week number calculator handles edge cases automatically so you can focus on planning, not arithmetic.
Results update in real time as you change inputs, so you can explore "what if" scenarios — adjust one variable at a time to see how it affects the outcome before committing to a purchase, plan, or decision.
This page includes step-by-step instructions, frequently asked questions, and practical tips below the calculator. Bookmark it for repeat use — many date time tasks come up weekly during projects, studies, or financial planning.
Common Uses
- ✓Project deadlines and sprint planning
- ✓Payroll, timesheets, and contract day counts
- ✓Travel scheduling across time zones
- ✓Personal milestones, birthdays, and countdown events
How to Use the Week Number Calculator
- 1
Enter a date
Select or type the date you want to find the week number for.
- 2
View week number
See the ISO week number, day of year, and quarter for that date.
💡 Tips & Tricks
- •Use week numbers for consistent scheduling in international teams.
- •The ISO week-year can differ from the calendar year at the start/end of the year.
- •Week 1 always contains January 4th — this is a quick check.
Frequently Asked Questions
What is the ISO week number?
How many weeks are in a year?
What day does an ISO week start?
Why do ISO week numbers matter?
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